Planar reflections as implemented in 4.12 are what I would consider the minimum to be usable. They don’t apply fog / atmosphere correctly, there aren’t any good controls for disabling expensive features in the reflection, dynamic shadows don’t work correctly in the reflection, and a few other things.
They work on translucency too - translucency will only apply the nearest planar reflection. If you disable Temporal AA, the downsampled planar reflections look a lot worse, they rely on the stability from temporal AA quite a bit.
Yes, you only pay for what’s enabled for the most part. There is one cost that can’t be toggled on the fly, that’s what the project setting to support the clip plane is enabling. This is because in D3D11 clip planes have to be implemented in the shader and not as separate graphics state.
You can use WPO for animation, but as it gets further from the plane, the less accurate the reflection will be.